Sensors based on solid-state spin systems offer high-performance, low-cost, low-power platforms for sensing or imaging of magnetic fields, electric fields, temperature, pressure, and other physical quantities, sometimes with resolution down to the nanoscale. A solid-state spin sensor employs color center defects (e.g., nitrogen vacancies), which are point-like defects in a solid-state host (e.g., diamond or silicon carbide), to measure physical quantities. The color center defects have quantum spin states that can be manipulated by optical and microwave radiation. Their quantum spin states can be made to be sensitive to certain physical parameters, such as magnetic field, and emit fluorescent light whose intensity depends on the defects' quantum spin state(s). The value of the magnetic field or other physical quantity to be measured is given by the energy levels of the quantum spin states of the color center defects or by the distribution of the color center defects between the different quantum spin states.
In some implementations, e.g., in magnetic field sensing, it is desirable to apply a bias magnetic field to the solid-state spin sensor. The bias magnetic field shifts the different quantum energy levels so that each quantum energy level can be distinguished from the other quantum energy levels, allowing each quantum energy level to be individually interrogated. Put differently, an appropriate bias magnetic field allows the color center defects' different quantum energy levels to be addressed and manipulated individually without affecting or manipulating the population residing in other quantum energy levels.
Under a suitable magnetic bias field, microwave and/or optical excitation radiation is applied to the solid-state spin sensor so that the physical quantity to be measured influences either the quantum energy levels or the distribution of the color center defects between the different quantum energy levels. The optical excitation and/or microwave radiation manipulates the quantum states of the color center defects in order to make a measurement of the physical quantity.
The color center defects emit fluorescent light in response to the optical excitation radiation and the microwave radiation. The value of the physical quantity to be measured can be inferred from the quantity of the detected optical fluorescent light. The amount of detected optical fluorescent light typically depends in part on the color center defect's quantum state, which in turn depends on the value of the physical quantity to be measured.
An Example Solid-State Spin Sensor
FIG. 1A shows a schematic of a standard solid-state spin sensor 100. A fixed value bias magnetic field 171 is applied to a solid-state sensor 110, which comprises a solid-state host with an ensemble of color center defects. The bias magnetic field allows the different quantum energy levels of the color center defects to be addressed and manipulated individually without affecting or manipulating the population of color center defects residing in other quantum energy levels. The fixed value bias magnetic field 171 is created using either one or more permanent magnets or using one or more wires (not shown) through which current is driven by a fixed value current source. In some implementations, the wires are arranged in one or more loops to create the bias magnetic field.
Microwave radiation and optical excitation pulses are applied to the solid-state spin sensor 110 with a microwave radiation source 130 and optical radiation source 120, respectively. A computing device 160 is used to control aspects of the microwave and optical excitation pulses, such as the power and spectral content. When excited by the optical excitation radiation, the color center defects in the solid-state spin sensor 110 emit fluorescent light 113, which is collected and sensed by a light detector 140. The output of the light detector 140 is digitized by an analog-to-digital converter 150 and sent to a computing device 160. Given the known temporal and spectral properties of the applied microwave radiation and optical excitation radiation, along with the detected optical fluorescent light 113, the computing device 160 can calculate the value 161 of the physical quantity to be measured.
FIG. 1B illustrate a measurement control sequence 101 for measuring an external magnetic field experienced by the color center defects in the solid-state spin sensor of FIG. 1A. The spins of the color center defects are initially prepared in a certain quantum state by illuminating the color center defects with a pulse of optical radiation 121 from the optical radiation source 120. Thereafter, the microwave radiation source 130 applies a first pulse 131 of microwave radiation to the color center defects. This transfers the color center defects into a superposition quantum state. The measurement is arranged so that the physical parameter (here, external magnetic field) to be sensed affects the rate of phase accumulation during a precession period during which the spins of the color center defects precess. After the precession period, the microwave radiation source 130 applies another pulse 133 of microwave radiation to the color center defects, further manipulating the color center defects into quantum states. After the second microwave pulse 133, the light source 120 excites the color center defects with another optical pulse 123, e.g., at a wavelength of 532 nm. The color center defects fluoresce in response to the second optical pulse 123, e.g., in a wavelength band from 637-850 nm. The amplitude modulation of the fluorescent light encodes the value of the magnetic field sensed by the color center defects.
Generating a Static Bias Magnetic Field in a Solid-State Spin Sensor
To date, high-performance ensemble solid-state spin sensors that use a bias magnetic field for operation employ one of two methods to create the bias magnetic field. FIG. 2A shows one method, denoted “Method A,” which employs one or more permanent magnets 270a and 270b (e.g., NdFeB, SmCo, Alnico, ferrite, or some other permanent magnetic material) to create a static bias magnetic field, shown in FIG. 2B (simulation). In this example, the permanent magnets 270a, 270b are ring-shaped and create axially symmetric magnetic field lines.
In some implementations, the permanent magnets 270a and 270b are close to a solid-state host 210 doped with color center defects (e.g., NVs in diamond). For example, one permanent magnet may be located 10 cm away from the solid-state spin sensor. In another example, two magnets (e.g., magnets 270a and 270b in FIG. 2A) may be located 15 cm away from the solid-state spin host 210. In another example, magnetic material may be deposited (e.g., via sputtering) directly on or very close to (e.g., within 1 cm of) the solid-state spin host 210 for implementing chip-scale devices.
FIGS. 3A and 3B show another method, denoted “Method B,” of generating a bias magnetic field. This method employs one or more sections of wire (here, two circular wire loops 370a and 370b on opposite sides of a solid-state host 310 doped with color center defects) and a current source 380 that drives a fixed amount of current through the wires 370. This fixed current creates a static bias magnetic field. FIG. 3B shows a radially symmetric simulation of the static magnetic field lines generated by the fixed current running through the coils 370a and 370b. There are many possible wire configurations, the most common of which is known as a Helmholtz coil and employs two circular clusters of wires (e.g., as in FIG. 3A). An alternative configuration, known as a Maxwell coil, uses three clusters of circular loops of wires.
It is also possible to create a static bias magnetic field using a combination of Method A and Method B. For example, the bias magnetic field can be created using a Helmholtz coil configuration of wires driven by a fixed value current source in addition to a single permanent magnet.